Hi,
I am sorry you are experiencing this.
Working your contracted hours is lawful.
Being criticised, marginalised, or bullied because you are part-time is not.
You are not being unreasonable. The reduction in hours was approved by your employer and as such, how you are being untreated is unfair, especially if you are fulfilling your duties within those hours.
I am not sure if I have missed what you do for work, apologies. If this situation continues or escalates, you’ll need to consider whether it’s sustainable long-term. Unfortunately, workplace cultures like this often don’t change.
If you are being impacted by this, you must log every incident of this happening in as much detail as possible (names, dates, what has been said, witnesses). You must also raise these concerns to your own manager, or their manager / HR, in writing, to ensure there is a formal record. This helps protect you if the situation is later reframed as a performance or capability issue.
If the workload cannot reasonably be completed within your contracted hours, that is a management and resourcing issue, not a personal failing. If duties have increased or not been adjusted to reflect your hours, that is relevant and should be documented.
Escalation options
A) Phone ACAS (free) and speak to an employment solictior (many offer free intial consultations).
B) Raise a formal grievance with all details, including names, dates etc.
C) Submit a Subject Access Request. This requires your employer to disclose all personal data held about you, including emails, messages, teams, and notes where you are identified. If people are writing anything about you, it will have to be disclosed. There is quite a lot of information available for free online.
Finally, protect yourself by keeping discussions about this issue limited to HR or your manager (provided they are not involved). Avoid informal conversations with colleagues about it.
You are entitled to work your contracted hours without being treated as “less than” for doing so.